The Singapore government statement says that al Qaeda leaders "showed interest" in the attack plans, which were described as "developed and ready for action." But for "reasons not known" the plan was never carried out.

A second plan called for an attack on U.S. Navy vessels in the waters of Singapore, with a "kill zone" designated on a confiscated defense military map.

The ministry's press release also said that the group was planning attacks on the British High Commission, the Israeli Embassy, the Australian High Commission in Singapore and commercial buildings housing U.S. companies.

Another suspect took photos of Singapore's Paya Lebar Airbase and U.S. military aircraft at the base as a "potential target for [a] terrorist attack," the government said.

It said that some of the detained suspects were approached by two men in September and October last year to "assist in a plan for terrorist bombing against specific targets in Singapore."

The terrorist cell had about four tons of ammonium nitrate, an explosive chemical, in Malaysia and had been instructed by two 'foreign suspects' to get 17 more tons to make truck bombs, the statement said.

On of the two men referred to as "Mike" was "a trainer and bomb maker" with the Moro Islamic Liberation Front, some of the detainees told authorities. The Moro Islamic Liberation Front is a Muslim group trying to set up an independent state in the southern Philippines.

The other man, "Sammy" was of "Arab extraction", the government said.

Security alert

Authorities say that all 13 detained suspects belong to Jemaah Isamiya, an organization that has cells in Singapore, Malaysia and Indonesia. Its leader is still at large.

Pentagon officials said that the break which led to the arrests underscores the importance of the intelligence gathering mission of U.S. troops in Afghanistan.

"Almost every arrest leads to additional pieces of information. It may be scraps of information in their pockets, it may be things they say, it may be other connections that occur," U.S. Defense Secretary Donald Rumsfeld said.

American warships have continued to make port calls in Singapore over the last several weeks with extra security measures.

U.S. Navy sources say they were confident that Singapore's government had taken steps to control security at the port and key areas frequented by U.S. sailors.

Security throughout the Southeast Asian nation has also been beefed up, particularly around U.S. companies and interests.

Singapore is home to about 17,000 Americans and about 6,000 multinational companies have their regional headquarters in the city-state.